This invention relates to helicopters and more specifically to air-driven helicopters in which a flow of air exiting from the rotor blades near the tip is used to provide the necessary rotation of the rotor blades.
The most common construction for helicopters, which historically has been used and which generally is employed both in commercial and military helicopters at the present time, is to cause the rotor of the helicopter to rotate by mechanical interconnections of the rotor to an engine through various types of mechanical transmissions. Several disadvantages are inherent in such conventional systems, whether an internal combustion engine is used as the primary power source or whether a turbine engine is employed. A primary disadvantage is that significant high torque loads are placed on the structure which is used to support the vertical shaft connecting the rotor blades with the engine. This torque must be counteracted to prevent a counterrotation of the body of the helicopter with respect to the rotor. Typically, this is accomplished by the provision of an additional small tail rotor which is mounted in a vertical plane with its hub at right angles to the fore-aft axis of the helicopter body. The force which is applied to the body of the helicopter by the tail rotor is controlled by coordinating the pitch of the tail rotor with the driving power applied to the main rotor blades in order to obtain stable operation of the helicopter. An enormous amount of stress is placed on the body members of the helicopter fuselage as well as the transmission components used to interconnect the engine with both the main rotor and the tail rotor.
It is readily apparent that a large number of rotating and moving parts exist in a conventional helicopter in order to drive and control the two rotors. This is a significant disadvantage because many bearings, operating under significant stress (rotational, centrifugal and the like), must be employed. These bearings and the other moving mechanical parts are costly and require frequent and expensive maintenance. In fact, the maintenance and repair hours generally exceed the actual flight hours of such a helicopter. As a result, maintenance is a significant cost factor to be considered for the operation of a conventional helicopter.
Various attempts have been made to overcome the disadvantages of conventional mechanical drive train mechanisms. One approach is to place a jet engine or turbine at the end of each of the rotor blades. This removes the structural requirements placed upon the vertical rotor shaft in conjunction with the interconnection of the rotor to an engine located within the body of the helicopter. The hub of such a rotor-tip jet-turbine driven helicopter then can comprise a simple rotating disc or the like with its center at the vertical rotor support shaft. Significant fuel delivery problems, however, exist for supplying fuel from the body or fuselage of the helicopter up through the rotor support shaft and through the hub to the rotating blades. This presents an extreme safety hazard because of the high volatility of the fuel; and leaks between the hub, the non-rotating rotor shaft, and rotating rotor blade are difficult to prevent. Furthermore, the centrifugal force acting upon the fuel due to the rotating rotor blades changes depending upon the speed of the blade. This can result in either a too rich or too lean fuel mixture supplied to the engine. In addition, the jet engines must breath their own exhaust. Consequently, power failures occur with such helicopters unless complex control mechanisms are provided for controlling the fuel supply to the engines. Patents directed to such rotor tip jet driven helicopters are the U.S. Pat. Nos. to Root, No. 2,931,441; Boushey, No. 2,917,895; Lindenbaum, No. 3,482,803; Ranson, No. 2,984,304; and Gose, No. 3,010,678.
To take advantage of the simplified structural requirements of the rotor blade acting as a simple rotating disc, but without the problems of conveying volatile fuel to jet engines mounted on the tips of the rotor blades, various designs utilizing the flow of pressureized air delivered through a hollow rotor shaft to hollow blades have been developed. In systems using this general design, a flow of air passes through the rotor blades to nozzles or air reaction engines located at the tips of each of the blades and directed rearwardly. Consequently, air discharging through the nozzles results in reactive forces in the opposite direction to rotate the blades about the hub. A variety of attempts to develop practical helicopters using this concept of an air-driven rotor have been made in the past.
A recent air-driven helicopter system is disclosed in the patent to Nagler, U.S. Pat. No. 3,830,588. This patent discloses an air driven helicopter which has a hollow rotor shaft connected to an air compressor to convey a flow of air through the rotor shaft to the rotor hub. The hub rotates about the fixed shaft on ball bearings, with the enter line of the rotor blades concentrating a center of force which is primarily above the bearings. Separate air seals and a separate spherical bearing to handle the tilt of the rotor blades is required. In addition, flexible bellows are provided to accommodate the tilt functions of the rotors effected by the swash plate. Because of the relatively high temperature of the compressed air which is used in such a helicopter, the ball bearings and the air seals of the system disclosed in this patent require frequent maintenance. In fact, the high temperatures to which the ball bearings are subjected cause them to have very limited life.
Other prior art patents for air-driven helicopter rotors, which are subject to the same disadvantages present in the Nagler system described specifically above, are the U.S. Pat. Nos. to Laufer, No. 2,845,131; Andrews, No. 3,119,577; Leoni, No. 3,370,809; Chaney, No. 3,259,195; Fischer, No. 3,186,491; British Pat. No. 938,459; French Pat. No. 1,002,007 and Italian Pat. No. 419,603.
In an effort to overcome the disadvantages of short bearing life and high maintenance in air-driven helicopter rotors of the type disclosed in the Nagler patent, a helicopter rotor hub system employing high temperature graphite bearings in place of the ball bearings and separate seals of Nagler has been developed. This system is disclosed in the patent to Henry, U.S. Pat. No. 4,473,335. The rotor hub of the Henry helicopter comprises a section of a spherical stainless steel ball attached to the end of a hollow rotor shaft. As in Nagler, an air compressor is located within the helicopter fuselage to supply air to the hollow rotor shaft. This air discharges through the shaft to the hollow rotor blades which have discharge nozzles on the trailing edges of their tips. The sole support bearing for the blades is in the form of a cup-shaped concave graphite bearing which engages the stainless steel ball. The rotor blades are attached to this bearing. The high temperature graphite which is employed provides a lubrication-free bearing surface and also provides a seal against air leakage.
While the hub arrangement of the Henry Patent overcomes some of the disadvantages of air-driven helicopters, such helicopters, including those constructed in accordance with the Henry patent, are still less than half as efficient as a shaft driven helicopter. This is unacceptable for most uses because of the high cost of the fuel consumed and the necessity for storing large amounts of fuel (which inherently results in added weight) in the helicopter fuselage. The efficiency losses occur because of the relatively long path which must take place from the compressor located within the fuselage up through the support shaft and out through the entire lengths of the blades to the tips which limits the practical volume flow rate of air. Also, heat is lost from air along the path, reducing thrust, and the air must be accelerated to tip speed as it moves outward along the blade.
Consequently, it is desirable to provide an air driven helicopter which overcomes the disadvantages of prior art helicopters of the various types which have been discussed above. Preferably, a helicopter should be driven by air thrust at the tips of the rotor blades but without the necessity of ducting either fuel or air up through the support shaft and the blades to the blade tips. In addition, the number of moving parts in the helicopter should be reduced as much as possible in order to reduce the maintenance required.